Before GNOME 42, Firefox matched the variant (light or dark) of the theme that I was using. Now this doesn’t happen any more. Does this have to do with the fact that Firefox isn’t yet fully compatible with GNOME 42?
I use the Night Theme Switcher extensions (Night Theme Switcher - GNOME Shell Extensions) but even disabling the extension doesn’t change this behaviour. Firefox theme can be set either to dark or to light version, but it will stay that way.
Night Theme Switcher can change both the system theme and legacy application theme. In Night Theme Switcher settings, make sure Themes > Switch GTK Theme is enabled with a corresponding theme for the Day and Night variant.
Another option:
Firefox will use the legacy application theme as it’s a Gtk 3 application.
Thank you. The things is that the Legacy theme switcher (which I have installed and enabled) doesn’t seem to change Firefox’s or other apps’ theme. GTK3 apps will use whatever theme I have selected in Tweaks > Appearance for Legacy Applications, but that theme won’t change with day or night time.
Does the theme change toggling between Light and Dark in Settings > Appearance? It works for me with Adw / Adw-dark.
I re-installed the extension, re-loaded the system and now it works. 
Mistery, but problem solved.
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